Princeton, New Jersey (CNN) -- In his first year in the White House, President Obama has proved to be an elusive figure. This is ironic given that his campaign to win the Democratic primary in 2007 and 2008 had been premised on the idea that voters preferred a candidate who stood for something.

For one thing, he distinguished himself from Sen. Hillary Clinton by highlighting the fact that he had been against the Iraq War from the start and never wavered in his position.

Yet in 2010, many Democrats, as well as Republicans, are unsure of who President Obama is and what exactly he stands for.

The president has opened himself up to attack by falling into the trap of being the pragmatic president who avoids taking clear stands. He makes himself vulnerable to attack from liberals who believe he is disloyal to their cause, moderates who are not convinced that he really will govern from the center and conservatives who can paint him as embodying their very worst fear.

Several times, the nation has seen Obama take a firm stand but then back off. Most recently, the president has wavered on his decision to have one of the masterminds of 9/11 tried in a civilian court in New York. When a backlash developed from New York officials, the Obama administration backed down and said it was reconsidering.

This was similar to what happened with the Guantanamo Bay Naval Base. The president started his term announcing he would close the facility within a year. But from the time of the announcement, he encountered fierce opposition from members of both parties who didn't want detainees transferred to detention centers in their home states. At this point, the schedule to close Guantanamo remains unclear.

We have seen the same kind of elusiveness on domestic policy. With some exceptions, such as his effective speech on health care after a politically brutal month, the president has avoided making too many speeches where he outlines his broader vision of public policy.

At several points in the health care negotiations, the president has conceded on significant items that he once supported. Many liberals were upset that he gave up on the public option and allowed for abortion restrictions to be placed in the bill. Many moderates were unhappy that the president did not fight harder for cost-control measures that he once said were essential to the success of the legislation.

When presidents fail to define themselves, they can find themselves in political trouble. To be sure, it is essential that a president avoid being so ideologically rigid that he circumscribes the opportunities for productive negotiations.

But at the same time, a president needs to let voters and fellow politicians know what he is about. Lyndon Johnson frequently spoke about his broader vision for a Great Society, even while accepting significant compromises on the particulars of bills.

Ronald Reagan stuck to his arguments about the dangers of too much government and the need for an aggressive stance against Communism, even while accepting policies, such as arms negotiations with the Soviets in 1986 and 1987, that signaled very different goals.

Even Bill Clinton, who was notoriously opportunistic, was able to define himself as a centrist Democrat who would defend the social services provided by the federal government since the New Deal while pushing for market-based reforms and market deregulation.

Then there were presidents who proved much less popular.

Jimmy Carter ran into trouble in 1979 and 1980, as critics from all sides of the political spectrum saw in him a lot of what they didn't like -- conservatives saw a left-wing Democrat, liberals saw a hidden conservative -- while few people found something in his presidency for which they were willing to fight.

Similarly, George H.W. Bush saw his political standing plummet after a high point during Operation Desert Storm for, among other things, refraining from dealing with the "vision thing" on domestic policy during a recession. As a result, he found himself under attack from multiple angles.

Since the Republican victory of Scott Brown in Massachusetts, Obama has tried to speak out in a more forceful fashion but continues to send signals in many directions.

At some moments, he draws on a populist rhetoric to attack the banks and call for regulations; at other times, he taps into the agenda of the moderate Democratic Leadership Council with his calls for deficit reduction and spending cuts.

Most recently, the president has invited Republicans to participate in a summit on health care. If the event comes off, it will be important that Obama provides a strong sense of what provisions within the existing legislation the White House will insist on if there is to be a final deal.

The president is still early in his presidency. He has time to correct his problems and to emerge stronger during year two. But time is slipping away. Many members of the administration must feel like athletes on the court, looking up at the game clock as the minutes fade away.

The president must do better at explaining just what his presidency is about. This does not mean abandoning a strategy of negotiation and compromise and ideological flexibility, but it does mean better defining the person who will be at the negotiating table. Otherwise, everyone else in the room will do that job themselves.

The opinions expressed in this commentary are solely those of Julian Zelizer.